1. Field of the Invention
This invention relates to a particle size measuring apparatus for optically measuring the particle size and/or particle size distribution of particles.
2. Description of the Related Art
Among the prior art methods, there is a method measuring light intensity scattered by one particle to be measured. This method is usually called the Coulter counter. A prior particle size measuring apparatus embodying this method is described in A. Ederhof, and G. Bibelius, "Determination of Droplet Sizes and Wetness Fraction in Two-phase-flows Using a Light-scattering Technic", Institution of Mechanical Engineering Journal of Mechanical Engineering Science, 1976, pages 21 to 27. Similar apparatuses have been disclosed in Japanese Patent Disclosure No. 78-13625, 81-58638 and 81-145330.
FIG. 1 shows the particle size measuring apparatus in the above paper. The apparatus includes an optical system for irradiation comprising light source 1, optical fiber 3, aperture 15, and objective lens 5 and an optical system for receiving light comprising objective lens 7, prism 16, aperture 8, and optical fiber 9. The two optical systems are arranged in such a manner that the angle between the optical axes is 90.degree. at the point P. The light emitted from light source 1 and transmitted through optical fiber 3 is irradiated to aperture 15. The image of aperture 15 is formed at the point P by means of objective lens 5. The light scattered at an angle of 90.degree. by a particle existing at the point P is collected by objective lens 7 and turned at an angle of 90.degree. by prism 16. The objective lens 7 forms an image at aperture 8 at the point P.
The scattered light through aperture 8 is guided by optical fiber 9 to photomultiplier 10. The photomultiplier 10 converts the scattered light to the electric current signal. The electric signal thus produced is fed to waveform analyzer 17 where the electric signal proportional to scattered light is inverted to the particle size (diameter). The oscilloscope 18 monitors the electric signal.
As shown schematically in FIG. 2, the point P is, in fact, a cube with some dimensions which are defined by the images of apertures 15 and 8 and constitutes the measuring volume.
When a particle flows through this measuring volume in the direction perpendicular to both the two optical axes of objective lenses 5 and 7, the light scattered by the particle at an angle of 90.degree. is received by the optical system for receiving light and is guided into photomultiplier 10 and is converted into an electric signal.
This electric signal is in a form as shown in FIG. 3. The pulse width corresponds to the flow speed of the particle and the pulse height to the particle size (diameter). To put more specific, the pulse width increases as the the particle flowing speed decreases and the pulse height becomes higher as the particle size increases. Hereafter, this pulse is referred to as the scattered light pulse.
The intensity of the scattered light at an angle of 90.degree. by a particle of a known size can be calculated by Mie's theory or Fraunhofer's diffraction theory. Therefore, if the relationship between particle size and scattered light intensity is obtained by calculation beforehand, it is possible to determine particle sizes from the scattered light pulse height measured by waveform analyzer 17.
In the above prior apparatus, however, the intensity distribution of the irradiated light is in the form of normal distribution since light is irradiated through optical fiber 3 to the measuring volume. Thus, it is difficult to illuminate the measuring volume uniformly. That is to say, the light intensity is higher toward the center of the measuring volume and it decreases toward the peripheral area.
Hence, this prior apparatus has a drawback that the scattered light pulse height varies whether the particle flows the central area or the peripheral area of the measuring volume. The drawback results in errors in particle size measurements.
To avoid this problem, it is necessary to guide particles to be measured via a very thin pipe and finely control the flowing position of the particles so that the particles flow an area (central area) in the measuring volume where the intensity of the projected light is relatively uniform. However, this control itself is no easy matter and if this control is implemented, it is impossible to use the whole area of the measuring volume, and the number of particles that can be measured per unit time decreases notably. Consequently, it takes a long time to obtain particle size. And, to obtain a particle size distribution is virtually impossible.
As described above, with particle size measuring apparatuses of the prior art, measurement errors occur depending on the flowing position of the particles in the measuring field of view. If the flowing position is limited to the central area to solve this problem, long time has to be taken for measurement.